In order to provide a satisfactory visual display on the screen of a cathode ray tube system which utilizes a plurality of electron guns, it is necessary that all of the guns have similar characteristics, and in particular that the emission currents from the several guns not vary by an appreciable amount from each other under normal operating conditions. The electron emission from the individual guns are tested by what is known as an emission test in which each gun or tube is individually biased near cutoff and then the bias on all guns or tubes is reduced by an identical amount resulting in a substantial amount of emission current from each gun or tube. For proper tracking, manufactures recommend that the ratio of the emission current from the highest current gun to the lowest current gun not exceed 1.5 to 1. Experience also shows that any ratio greater than 1.5 to 1 results in unacceptably poor tracking and poor picture reproduction.
In patent 3,688,184--Baum there is described a method and circuit for determining if a multiple-gun cathode ray system has satisfactory tracking characteristics. It would be desirable, however, to determine which of the guns has an unacceptably low cathode emission in those cases where the tracking characteristics of the system are unsatisfactory.